"She tried to cradle Katie's head and the officer led both parents away in considerable distress. Paramedics arrived quickly but there was little they could do.

"They saw a large incise wound, 20 centimetres from her sternum to her stomach. It had cut through fat, skin and muscle.

"After working on her body for several minutes Katie was taken to hospital.

"She was reported to have no heart output but resuscitation attempts continued until they arrived. However, attempts to revive her failed and she was certified dead at 5.44pm."

The court was also told how the teenager - who was 15 at the time of the killing - was found in the street by a member of the public after the incident.

Alison stands on the steps outside York Minster after her daughter's funeral (Image: SWNS)

She was on the phone to the police and had told the operator Katie was dead and she didn't know where she was, the court was told.

She also asked the man if he knew where they were.

The man then found Katie lying on a nearby piece of land with a cut to her neck, Mr Reeds told a judge sitting in the courtroom.

She appeared to have no pulse or signs of breathing, he said.

Post mortem results showed Katie died from asphyxiation, with her wounds inflicted after her death, the court was told.

Pathologists found that a gloved hand was held over her face to suffocate her before she had her neck and torso slashed.

The autopsy revealed a six-centimetre slash wound to the young girl's neck and a similar 20-centimetre cut to her torso, the court heard.

Pathologists found that a gloved hand was held over Katie's face to suffocate her before she had her neck and torso slashed (Image: SWNS)

The latter injury had cut through fat, skin and muscle.

But the prosecutor said neither of the cuts were found to have caused her death. He said Katie had been smothered before the injuries were made.

He added that the defendant - who has a history of depression and self-harm - has been subject to four psychiatric and psychological reports.

The court heard the teenager posted a picture to Instagram of self-harm wounds to her arms just two days before the incident.

The prosecution also referred to her delusional beliefs - among which was the thought the other people were "robots" and "not real".

The court head police recovered a number of items from the defendant's home.

These included drawings of stick-men in various poses depicting killing and death, and a reference to "they are not human".

Mr Reeds said a report on the teenager had found she was having thoughts that other people weren't human and that they were robots.

Katie loved the outdoors and reading (Image: SWNS)

"She got distressed when asked whether she had killed Katie to test whether she was a robot," he told the court.

He added: A search of her bedroom recovered a hooded top sliced over a pillow then slashed. There were also notes and books and comics which are described as being of a violent nature.

"Police later recovered a soft toy - a Simba toy - which had its ears cut off and stuffed into the stomach of the toy through a vertical slash."

Mr Reeds said it was around 4.20pm that a local returning from walking her dogs saw Katie and the teenager seemingly playing on the grass.

"She saw the defendant and Katie on the Square," he said.

"Both girls appeared to be playing on the grass.

Mourners wore bright scarves and ties and a number of young children wore rainbow-coloured hair bows (Image: PA)

"As she got closer both girls got up and Katie didn't appear to be in any distress. She thought they were just two girls playing and it didn't cause her any concern.

"At around 4.35pm her husband Peter Mills was returning home by car. He saw the defendant standing on Alness Drive. She was distressed and covered in mud.

"His first reaction was that she had been the victim of a serious attack. She was on her phone and she asked Mr Mills if he knew where they were.

"He didn't know she was on the phone to the police. She had called 999 and told the operator Katie was dead and she didn't know where she was.

"The defendant did in fact tell Mrs Mills that Katie was on the field. She (Mrs Mills) said she wasn't crying but looked pale and drawn and didn't look well at all.

"As Mrs Mills took the girl into her house for safety, Mr Mills ran to the Square. He saw Katie around 15 to 20 yards beyond the entrance to the field.

"She was lying on her back, her hands to the side and above her back. Her face was covered in blood. He attempted to find a pulse or signs of breathing but with no success.

Katie was "lifeless" and lying on her back, the court heard (Image: SWNS)

"Katie was lifeless. She didn't respond to him talking. He covered her with his coat and ran back to the house and got his wife to call an ambulance.

"In the meanwhile, Alison Rough now began to be concerned by Katie's absence and walked down to the Square shouting. There was no response."

The court heard Mrs Rough dialled 999 at 4.47pm - 13 minutes after the defendant called the police - to report Katie missing.

The operator told her there had been a report of a crime at the field and she immediately returned to Alness Drive with her husband.

Mr Reeds added: "Mrs Mills told them: 'She's on the field.' Both of them ran down to the field. An officer was just in front of them. He started chest compressions but Katie's position revealed a substantial slash wound to her neck that was particularly deep."

The prosecutor said the defendant was detained at the Mills' house.

"She appeared upset and was crying and the officer who arrested her also thought she was in shock," he said.

"All she would say is she had been on the field. She was asked if she had been in an argument, she said no. She was then asked if she had any weapons.

Forensics at the scene in York (Image: SWNS)

"She produced a red Stanley knife from one pocket and two spare blades were found in another pocket when she was searched.

"A blood-stained green glove was found in the pocket of her hooded top. The Stanley knife was blood-stained and it later tested positive for Katie's DNA."

Mr Reeds told the court there was no dispute that the defendant's mental health problems meant she was suffering from diminished responsibility at the time she killed Katie on January 9, even though the killing was pre-planned.

Katie's grieving parents sat in the courtroom, surrounded by family members, as the teenager entered her plea today.

The couple had been earlier pictured arriving at the building via car.

The teenager sat next to a solicitor and looked at the floor as she pleaded guilty to manslaughter by diminished responsibility via videolink.

Her solicitor confirmed her name when asked by the judge, Mr Justice Soole.

Nicholas Johnson QC, defending, asked the court if the charge of murder could be put to the girl again and she wrote her plea on a piece of paper.

Her solicitor told the court: "I can confirm she has indicated not guilty to murder but guilty to manslaughter."

Mr Reeds said: "We are going to accept that plea of manslaughter by diminished responsibility."

The teenager wore a black hooded top for her court appearance.

In February, hundreds of mourners donned rainbow-coloured accessories as the youngster's life was celebrated at her funeral.

Around 400 people packed into York Minster for a service led by the Archbishop of York, Dr John Sentamu.

Dr Sentamu told the congregation that he sat with Katie's coffin overnight after realising she would be alone following a family service at his Bishopthorpe Palace residence.

Mourners wore bright scarves and ties and a number of young children wore rainbow-coloured hair bows after they were asked by Katie's parents to dress formally but with a colourful twist.

Several family members gave tearful Bible readings, while Katie's uncle, Steven Rough, paid an emotional tribute to his niece.

He said: "We should focus on how happy she made us when she was in our lives."

Mr Rough described "daddy's girl" Katie's love of the outdoors and reading, particularly poetry and Dr Seuss. He also talked about her family's memories.